Genetic basis for a predisposition to cancer has been demonstrated by the increased incidence in people with mutated P-53. That some genetic alterations may result in protection is suggested by altered incidences of some tumor types in Cystic Fibrosis. For example, there were fewer than expected cases of breast cancer. We have developed an animal model for investigating tumor growth in CF, the CF nude mouse. We have shown that an aggressive human breast cancer has diminished takes and reduced growth rate. The phenotype is associated with elevation in blood ATP. In vitro and in vivo studies have shown that elevations of extracellular ATP retard tumor growth.